In situ X-ray absorption spectroscopy study of the thermal behaviour of giant magnetoresistance CoxAg1-x and NixAg1-x heterogeneous alloys

Heterogeneous CoxAg1-x and NixAg1-x alloys have giant magnetoresistance properties. Such alloys, with atomic concentrations x = 0.20 and 0.37, were studied by transmission X-ray Absorption Spectroscopy at the Co or Ni K-edge after in situ thermal annealing. For alloys as-deposited and annealed at 200 degrees C, Extended X-ray Absorption Fine Structure analysis displays both Go-Co (Ni-Ni) bonds related to Co (Ni) atoms agglomerated in magnetic particles and Co-Ag (Ni-Ag) bonds related to Co (Ni) atoms in substitutional sites in the Ag matrix. At the same alloy concentration, the miscibility in the Ag matrix is found larger for Ni than for Co. After annealing around 250 degrees C, the marked decrease of the Ag neighbour peak corresponds to a diffusion of magnetic atoms outside the Ag matrix. The Go-Co coordination number increases regularly with annealing temperatures up to 450 degrees C reflecting a progressive expansion of Co particles. On the contrary, for Ni alloys, no further particle expansion has been observed in the same annealing range. This different behaviour may be linked both to the difference between the as-deposited structural states and to the Co/Ag and Ni/Ag interfaces energies. Using a simple model, evolution of the mean particle size has been estimated as a function of annealing.